1. Technical Field
This invention relates to audio systems and, more particularly, to voice switching circuits which connect to an audio line for providing two-way voice switched communications.
2. Description of the Prior Art
The use of analog speakerphones has been the primary "hands free" means of communicating during a telephone conversation for a great number of years. This convenience has been obtained at the price of some limitations, however. Known speakerphones usually require careful and expensive calibration in order to operate in an acceptable manner. They are also designed to operate in a worst-case acoustic environment, thereby sacrificing the improved performance that is possible in a better acoustic environment.
The operation of conventional analog speakerphones is well known and is described in an article by A. Busala, "Fundamental Considerations in the Design of a Voice-Switched Speakerphone," Bell System Technical Journal, Vol. 39, No. 2, March 1960, pp. 265-294. Analog speakerphones generally use a switched-loss technique through which the energy of the voice signals in both a transmit and a receive direction are sensed and a switching decision made based upon that information. The voice signal having the highest energy level in a first direction will be given a clear talking path and the voice signal in the opposite direction will be attenuated by having loss switched into its talking path. If voice signals are not present in either the transmit direction or the receive direction, the speakerphone typically goes to an "at rest" mode which provides the clear talking path to voice signals in a receive direction thereby favoring speech from a distant speaker. In some modern analog speakerphones, however, if voice signals are not present in either the transmit direction or the receive direction, the speakerphone goes to an idle mode where the loss in each direction is set to a mid-range level to allow the direction in which voice signals first appear to quickly obtain the clear talking path.
Most high-end analog speakerphones also have a noise-guard circuit to adjust the switching levels according to the level of background noise present. Switching speed is limited by a worst-case time constant that assures that any speech energy in the room has time to dissipate. This limitation is necessary to prevent "self switching", a condition where room echoes are falsely detected as near-end speech. A disadvantage of this type of speakerphone is that no allowance is made for a room that has good acoustics, i.e., low echo energy return and short duration echoes.
One recently developed speakerphone includes an arrangement that does consider the quality of the acoustics in a room and adapts its operating parameters accordingly. This arrangement is disclosed in U.S. Pat. No. 4,959,857, which issued to R. H. Erving and R. R. Miller, II on Sep. 25, 1990. This arrangement includes a calibration circuit which measures the acoustics of the room by emitting a tone burst through a loudspeaker associated with the speakerphone and measuring the returned time-domain acoustic response with a microphone also associated with the speakerphone. The measured time-domain acoustic response in this arrangement is indicative of the dissipation of the echoes in the room over a predetermined time period during which the echoes must dissipate to an acceptable level. Such an arrangement has been found satisfactory when the speakerphone is operating in a small room, such as an individual office, where typically a small loudspeaker and a microphone having limited sensitivity are employed. In such an arrangement, the loudspeaker does not generate enough acoustic energy nor does the microphone have enough gain to detect those echoes within the room that persist for durations longer than the measured time-domain acoustic response provided by this arrangement.
Where it is necessary to conference a large number of individuals, for example, the audience in a large conference room or auditorium, the conference environment requires a high gain loudspeaker and a sensitive directional microphone such as are provided in the microphone arrangement disclosed in U.S. Pat. No. 4,311,874 issued to R. L. Wallace, Jr. on Jan. 19, 1982. Unfortunately, due to the increased gain in the loudspeaker over that in the arrangement discussed above, the loudspeaker has the ability to charge the room with a tremendous amount of energy, especially if such a room does not have good acoustics. And the increased sensitivity of the microphone enables the continuous detection of these echoes to a much lower level. Thus the echoes in the room may remain at a detectable level long after the allotted period for measuring the time-domain acoustic response has expired. As a result, the echo duration information measured by the speakerphone may not be sufficient in a conference environment to prevent the speakerphone from self-switching. It is thus desirable to provide an arrangement which does not suffer from this problem.